index – Help to understand mongodb to explain the results

I have an index on the f1 field. {f1: 1}

Run an explain on the following query

db.coll.find ()

says that the winning plan is a CollScan (expected)

However, for db.coll.find (). sort ({f1: 1})

the explanation says

"stage": "FETCH",
"inputStage": {
"stage": "IXSCAN",

I have difficulty understanding this because, even if the index is used for sorting, should not I have done an analysis of the collection to extract the documents to be sorted?

Thank you